Moisture found in cantaleaver overhang


Old 02-07-07, 06:37 PM
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Moisture found in cantaleaver overhang

Two days ago i noticed paint bubbling on the wall of one room in my basement. Upon investigating i found an ice buildup above the wall against the conrete foundation which was melting. I also noticed that the vapour barrier was only brought up to the top of the stud wall and not tied into the cantaleaver overhang which protrudes about a foot and a half over the foundation. In this area is a heat duct which supplies heat into my kitchen above the area. We have recently been having some colder weather.

So yesturday i removed the wet drywall to continue my investigation, followed by removing the ice and drying the water which was leaking down the wall. Next i removed the batt insulation which was put into the area between the joists and below the floor above in the canteleaver overhang area. In this area i found ice and water which was obviously leaching down the foundation wall.

I can only guess that the reason there is an ice buildup in the area is due to condensation from the lack of a vapour barrier. I recently put in a high efficiency furnace which may be adding to the problem since i can only assume it is putting out more heat then the old furnace. This has not happened in the last seven years since this room was finished.

Has anyone heard of large amount of condensation caused by something similar? Or does this sound reasonable? My cure at the moment is to attempt to melt the ice and clean it up as good as i can until the summer when i can remove the sheathing on the outside of the house to properly air seal it. And then i guess i will re-insulate and take the vapour barrier from the top of the stud wall in the basement back as far as i can reach into the cantaleaver area.

I would like someone to give there opinions on whether they think i am proceding properly, or if i should attempt something else. Please help.
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Old 02-07-07, 07:45 PM
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Cantilevers are difficult to seal and insulate, especially when ductwork is present. It is highly recommended that ductwork NEVER be installed inside exterior walls, and your cantilevered floor is basically a type of exterior wall. If there is any possible way to relocate that ductwork so that it is not in the canilevered area, I would recommend that as an important first step.

If I were faced with your situation, here is what I would do: Assuming that floor joists run headlong into the cantilevered area, blocking should be installed above your basement wall between each floor joist to completely close and seal that area off from the rest of the home. Rigid ISO insulation would be a good choice, fitting it tightly and sealing the edges with PL300 or expanding foam. Keep in mind that ISO foam cannot be left exposed in living spaces, and would then need to be covered with drywall, which can also be fit and glued to the ISO with PL300.

The ideal location for the vapor barrier would be under your subfloor (heated side of the wall), where it could be sealed to the rigid foam blocking you push up to meet it, between each joist.

If it is not possible to install a vapor barrier over the top of the cantilevered floor joists (which would involve removing the subfloor to expose the floor joists) then you must install rigid ISO insulation against the subfloor from below the cantilever. Remove any soffit that covers the bottom of the floor joists outside the home. Rigid ISO insulation should be fitted into each floor joist bay up against the subfloor. Again, edges could be sealed with PL300 or expanding foam. The interior edge of the rim joist could also be fitted with rigid ISO foam, and edges sealed. The cavity could then be filled with unfaced fiberglass batts. The entire bottom of the cantilevered area should be covered with rigid ISO insulation, using the least amount of pieces possible. You need to seal outside air OUT. (actually you are preventing any warm interior air from escaping (and condensing). Finally, a finish product would be applied over the ISO insulation- aluminum soffit, AC plywood, etc.

If for some reason you cannot remove the vent (which I assume is just in one of the floor joist bays?), then you would likely want to install unfaced insulation around the vent from underneath the cantilever (keeping the vent on the heated side of the vapor barrier), followed by as many layers of rigid foam (the vapor barrier) as will fit inside the floor joists below the vent, sealing all edges to prevent air infiltration.

You may wish to refer to the illustration on page 50 of the following .pdf file from an educational site (long download)

Hope some of this helps.

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